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The maker of blockbuster pain reliever Vioxx said Thursday that it is voluntarily withdrawing the drug because of new data showing it increases the risk of heart attacks and strokes.

A Vioxx tablet. By Mike Derer, AP

This is one of the largest-ever withdrawals of a drug, says the Food and Drug Administration's Steven Galson. Drugmaker Merck (MRK) says 84 million people worldwide have taken the heavily promoted drug, available in the USA since May 1999. About 2 million Americans are on Vioxx, Galson says. "We have been concerned and aware of the potential for cardiovascular effects for the last few years," he says. "This is not a total surprise."

The news hit Merck's stock hard. It fell 27% to close at $33, wiping out $27 billion in market value.

In August, an FDA-funded analysis involving Kaiser Permanente patients found more heart attacks and sudden cardiac deaths among people taking Vioxx than among those on Celebrex or on conventional nonsteroidal anti-inflammatory drugs (NSAIDs) such as ibuprofen and aspirin.

In November 2000, Merck published a study in The New England Journal of Medicine that found a higher rate of heart attacks in patients assigned to Vioxx than those assigned to naproxen (an NSAID sold as Aleve). Because of that finding, the FDA in April 2002 required a new warning on the Vioxx label.

Vioxx, Celebrex and Bextra are the three so-called COX-2 inhibitors on the U.S. market. Drugmakers say they treat arthritis pain but are less irritating to the gastrointestinal tract than traditional NSAIDs. At least two other COX-2 inhibitors — including Merck's Arcoxia, already sold in 47 other countries — are in the pipeline.

"It is important to note that the results of clinical studies with one drug in a given class are not necessarily applicable to others in a class," says Peter Kim, president of Merck Research Laboratories.

The implications of the Vioxx withdrawal for other COX-2 inhibitors aren't yet clear. Merck says the new data — from a three-year study comparing Vioxx with a placebo in 2,600 patients with colon polyps — showed a higher risk of heart attacks and stroke only after 18 months of use. None of the other COX-2 drugs have data for longer than a year, Galson says.

"It's too early for me to say right now how we're going to change our requirements for the future, but, obviously, we're going to be more interested in long-term data," he says.